Many types of devices today include a digital camera that can be used to capture digital photos, such as with a mobile phone, tablet device, a digital camera device, and other electronic media devices. The accessibility and ease of use of the many types of devices that include a digital camera makes it quite easy for most anyone to take photos. Many users also want to enhance their photos, such as by cropping them to improve the visual composition and quality of the photos, or for a different aspect ratio (landscape, panoramic, etc.). A digital image, such as a photo, can be cropped to improve the composition by removing unnecessary or distracting objects from the image, while also shifting the subject focus of the image. Although there are general rules for image composition defined by expert photographers, such as the rule-of-thirds for image cropping, the composition styles and preferences can vary from one user to the next.
Due to the many varying composition styles and preferences, conventional cropping techniques include manually designing cropping rules which can involve tedious parameter adjustments for cropping with the user-designed rules. The limitations of other techniques result in limited features of an image being determined, such as with saliency estimation techniques used to determine the most prominent or noticeable objects or regions of an image. Conventional techniques to compute a saliency map for an image only take into account global saliency, while disregarding local saliency. The global saliency techniques are designed to detect a single salient region of an image as salient object detection, which disregards local saliency and is limited in handling complex scenes of an image with multiple objects. A global saliency approach that attempts to identify the main subject or principal region in an image scene can help to remove unnecessary content from an image when cropped, but does not take into account the overall composition of the image, and the cropped image may result in a poor visual image.